The present invention relates to an image processing method and an X-ray computed tomography (CT) system. More particularly, the present invention is concerned with a method of wavelet-transforming an image, reducing noises in the wavelet domain, and then inverse-wavelet-transforming the wavelet-domain image so as to restore the image, and an X-ray CT system that performs the image processing.
In X-ray CT systems, X-irradiation/detection equipment that rotates in a gantry acquires transmitted X-ray signals, which represent a plurality of views, from a subject, and tomographic images are reconstructed based on the transmitted X-ray signals. The rotation of the X-irradiation/detection equipment may be referred to as a scan. The acquired transmitted X-ray signal may be referred to as scan data.
A reconstructed image has noises therein reduced. One of noise reduction methods is such that an image is wavelet-transformed, the resultant wavelet-domain signal has noises thereof reduced, and the resultant signal is then inverse-wavelet-transformed in order to restore the image. The noise reduction in the wavelet domain is achieved by replacing signal components, of which levels fall below a predetermined threshold, with 0s (refer to, for example, Japanese Unexamined Patent Publication No. 2002-133399 (p.3, FIG. 1 to FIG. 7).
The foregoing noise reduction method of simply replacing signal components, of which levels fall below a predetermined threshold, with 0s causes a restored image to suffer a shift in a CT number, deformation of edges, an abrupt change, or any other side effect.
An object of the present invention is to provide an image processing method for reducing noises without causing a side effect and to provide an X-ray CT system that performs the image processing.